1. Field of the Invention
The present invention relates to a flat display device, and more particularly, to an electroluminescence device (ELD) using organic and inorganic light emitting materials.
2. Background of the Related Art
Presently, many different types of flat display devices are being used. Among the most popular are liquid crystal displays (LCDs), field emission displays (FEDs), electroluminescence devices (ELDs), and plasma display panels (PDPs). In particular, ELDs are formed in such a manner that an electrode is attached to both sides of an electroluminescence (EL) layer consisting of a hole transport layer, an light-emitting layer, and an electron transport layer. The ELD belongs to a group of next generation flat display devices because of its characteristic wide viewing angle, high aperture ratio, and high chromaticity. ELDs may be divided largely into inorganic and organic ELDs according to materials used to form the EL layer. Inorganic ELDs produce light by accelerating electrons by a high electric field. The accelerated electrons collide with luminescent impurities and the luminescent impurities are driven into a high energy state. The excited luminescent impurities fall to a ground state and emit energy in the form of light. In organic ELDs, electrons and holes are injected from an anode electrode and a cathode electrode, respectively, make an excited pair, and fall from an excite state to a ground state, thereby emitting light. Accordingly, since organic ELDs require lower driving voltages than inorganic ELDs and do not require a backlight, organic ELDs can be formed of a thin structure having low power consumption characteristics.
Generally, ELDs can be divided into a passivation and active matrix ELDs according to a method for driving a display panel. Passivation ELDs have a relatively simple structure that includes an organic layer formed between two orthogonal electrodes. However, active matrix ELDs have a more complicated structure that includes a switching thin film transistor (TFT1) and a light-emitting thin film transistor (TFT2) for controlling emission of light in each pixel region and actively drives respective pixels.
An ELD according to the related art will be explained with reference to the following drawings.
FIG. 1 is a driving circuit of a related art ELD, FIG. 2A is a cross-sectional view illustrating a related art ELD, and FIG. 2B is a cross-sectional view illustrating problems of a related art ELD.
In general, the ELD includes an anode electrode formed on a transparent substrate, an EL layer formed at an upper portion of the anode electrode, a cathode electrode formed on the EL layer, and a packaging plate attached to an upper portion of the cathode electrode, the packaging plate opposite to the transparent substrate. A passivation ELD includes a transparent anode electrode arranged in a line as a belt form on a transparent substrate formed of glass material, a passivation layer is formed on an entire surface including the anode electrode, an EL layer formed by depositing a hole transparent layer, an emitting layer, and an electron transparent layer on the passivation layer, a cathode electrode formed as a belt form crossing the anode electrode on the EL layer, a packaging plate provided with absorber, the absorber fixed by a semi-transparent film, and an adhesive for attaching the transparent glass substrate to the packaging plate to oppose each other.
In FIG. 1, the active matrix ELD includes a gate line (G.L) and a data line (D.L) orthogonally crossing each other and having a plurality of pixels, thin film transistors (T1) formed in the respective pixel regions and thin film transistors (T2), an EL device connected to an output terminal of the thin film transistor T2, and a storage capacitor (Cst) connected to an output terminal of the thin film transistor T1. In addition, a voltage (VDD) is connected between an input terminal of the thin film transistor T2 and the storage capacitor (Cst). The thin film transistor T1 selectively applies an image signal of the data line (D.L) to the respective pixel regions by an injection signal of the gate line (G.L), and the thin film transistor T2 controls emission of the EL device that consists of an anode electrode, an EL layer, a cathode electrode.
In FIG. 2A, the ELD includes a transparent substrate 10 having insulating characteristics, a gate line (not shown) and a data line (not shown) orthogonally crossing with each other on the transparent substrate 10, a thin film transistor T1 (not shown) formed at a crossing point of the gate and data lines, a thin film transistor 7 connected to an output terminal of the thin film transistor T1 (not shown), an anode electrode 8 formed of a material such as ITO, for example, and connected to an output terminal of the thin film transistor 7 for emitting light, a passivation layer 9 formed on an entire surface including the anode electrode 8, an EL layer 12 formed on the passivation layer 9 by depositing a hole transport layer 1, an emitting layer 2, and an electron transport layer 3, a cathode electrode 14 of a metal material formed on the EL layer 12, a packaging plate 19 provided with absorber 16, the absorber fixed by a semi-transparent film 15, an adhesive 18 for attaching the transparent substrate 10 to the packaging plate 19 to oppose each other. The thin film transistor T1 (of FIG. 1) is connected not only to the thin film transistor 7 but also to the storage capacitor (“Cst” of FIG. 1) for selectively applying an image signal of the data line to the anode electrode 8 by an injection signal of the gate line.
In FIG. 2A, the packaging plate 19 and the glass substrate 10 are attached by an encapsulation method by the adhesive 18, such as epoxy resin, for example, in a sealed environment with inactive gases such as N and/or Ar, for example. However, since the EL layer 12 and the cathode electrode 14 are easily oxidized by reaction with oxygen, the devices are easily degraded.
To solve these problems, the absorber 16 is used to remove water in the display device, and to fill a space of the packaging plate and the substrate with inactive gases. The packaging plate 19 is formed of a material such as glass and/or plastic, for example. In addition, the absorber 16 is formed of a fine powder such as BaO, CaCO3, zeolite, silicagel, and alumina, for example. The absorber 16 is contained in the packaging plate 19 and attached by the semi-transparent film 15 formed of a light weight material such paper and TEFLON®, for example. It is necessary to form the absorber 16 uniformly. Since the semi-transparent film 15 is formed of a light weight material, any increase in the weight of the absorber 16 will cause the semi-transparent film 15 to become displaced downward, thus causing the absorber 16 to come into contact with the cathode electrode 14, as shown in FIG. 1B. The problem is attenuated when an area of the ELD increases. Accordingly, when a large amount of absorber material is contained in the absorber 16 in a wider packaging plate, the semi-transparent film will become displaced downward very quickly. Conventionally, the semi-transparent film 15 is spaced from the cathode electrode 14 by less than a few hundred micrometers. Therefore, any displacement of the semi-transparent film 15 or non-uniform distribution of the absorber 16 may degrade the cathode electrode 14, thereby reducing durability of the ELD.